Refrigeration systems incorporating a vapor compression cycle can be utilized for single-temperature applications, such as a freezer or refrigerator having one or more compartments that are to be maintained at a similar temperature, and for multi-temperature applications, such as refrigerators having multiple compartments that are to be kept at differing temperatures, such as a lower temperature (freezer) compartment and a medium or higher temperature (fresh food storage) compartment.
The vapor compression cycle utilizes a compressor to compress a working fluid (e.g., refrigerant) along with a condenser, an evaporator and an expansion device. For multi-temperature applications, the compressor is typically sized to run at the lowest operating temperature for the lower temperature compartment. As such, the compressor is typically sized larger than needed, resulting in reduced efficiency. Additionally, the larger compressor may operate at a higher internal temperature such that an auxiliary cooling system for the lubricant within the compressor may be needed to prevent the compressor from burning out.
To address the above concerns, refrigeration systems may use multiple compressors along with the same or different working fluids. The use of multiple compressors and/or multiple working fluids, however, may increase the cost and/or complexity of the refrigeration system and may not be justified based upon the overall efficiency gains.
Additionally, in some applications, the compressor and/or refrigerant that can be used may be limited based on the temperature that is to be achieved. For example, with an open drive shaft compressor, the seal along the drive shaft is utilized to maintain the working fluid within the compressor. When a working fluid, such as R134A, is utilized with an open drive shaft sealed compressor, the minimum temperature that can be achieved without causing leaks past the drive shaft seal is limited. That is, if too low a temperature were attempted to be achieved, a vacuum may develop such that ambient air may be pulled into the interior of the compressor and contaminate the system. To avoid this, other types of compressors and/or working fluids may be required. These other types of compressors and/or working fluids, however, may be more expensive and/or less efficient.
Additionally, the refrigeration systems may require a defrost cycle to thaw out any ice that has accumulated or formed on the evaporator. Traditional defrost systems utilize an electrically powered radiant heat source that is selectively operated to heat the evaporator and melt the ice that is formed thereon. Radiant heat sources, however, are inefficient and, as a result, increase the cost of operating the refrigeration system and add to the complexity. Hot gas from the compressor may also be used to defrost the evaporator. Such systems, however, require additional plumbing and controllers and, as a result, increase the cost and complexity of the refrigeration system.